The present invention relates to an electric double-layer capacitor, and more particularly to an electric double-layer capacitor which is excellent in charge-discharge cycle life and durability upon application of a high voltage, and has a large energy density.
Electric double-layer capacitors capable of high-current charging or discharging, are expected to apply uses such as electric vehicles, auxiliary powers or the like. Consequently, there has been a demand for realizing electric double-layer capacitors which can be rapidly charged and discharged, and are excellent in durability upon application of a high voltage and charge-discharge cycle life.
The energy accumulated in a cell of the capacitor is calculated according to the formula of 1/2.multidot.C.multidot.V.sup.2, wherein C represents a capacitance (F) per cell and V represents a voltage (V) applicable to the cell. Since the energy accumulated in the capacitor is proportional to a square of the applicable voltage V, it is effective to apply a high voltage to the capacitor in order to increase an energy density of the capacitor. However, in the case where the voltage applied is too large, there arise disadvantages such as decomposition of an electrolyte used therein.
Therefore, in the case of conventional electric double-layer capacitors using a non-aqueous electrolyte, the withstand voltage per cell is limited to about 2.4 V though the value of the voltage is varied depending upon kinds of solute and solvent used in the electrolyte. When a voltage as high as not less than 2.5 V is applied to these conventional capacitors, there are caused disadvantages such as rapid increase of internal series resistance, rapid decrease of capacitance upon use or the like.
Under these circumstances, an activated carbon as a main materials of positive and negative electrode sides of the capacitor, a separator, an electrolyte, a container and the like, have been studied in detail and on the basis of these studies, it has been attempted to apply a voltage of 2.5 V to 2.8 V to the capacitor. For example, in order to improve a durability of the capacitor, there have been proposed a method of subjecting an electrode composed of activated carbon which is obtained by activating a phenol resin, a petroleum coke or the like with KOH, to heat treatment in an inert atmosphere; a method of using sintered products of a phenol resin, a furan resin, a polyacrylonitrile resin or the like as a raw material therefor (Japanese Patent Application (KOKAI) No. 8-162375(1996)); a method of using porous aluminum as a current collector of the capacitor (Japanese Patent Application (KOKAI) No. 8-339941(1996)); or the like.
However, these conventional methods have problems and are more or less unsatisfactory. For example, in the above-mentioned method of heat-treating an electrode composed of activated carbon obtained by activating phenol resin, petroleum coke or the like with KOH, in an inert atmosphere, there arises such a problem that an initial capacitance of the obtained capacitor becomes lowered. Further, in the methods described in Japanese Patent Applications Laid-open (KOKAI) Nos. 8-162376(1996) and 8-339941(1996), the durability of the capacitors cannot be essentially improved. For these reasons, it has been substantially impossible to apply a voltage of not less than 3 V to a cell of the conventional capacitors. This inhibits the production of capacitors having a high energy density.
In order to enable the voltage of not less than 3 V to be applied to a cell of capacitors, in Japanese Patent Application Laid-open (KOKAI) No. 8-107048(1996), there has been proposed a capacitor using a negative electrode composed of a lithium-doped graphite obtained by contacting graphite and lithium foil, a positive electrode composed of activated carbon and an electrolyte containing lithium ions as a solute.
However, the negative electrode of the capacitor is a non-polarized electrode so that an oxidation-reduction reaction tends to be caused between the negative electrode and the electrolyte. Consequently, there arises a problem that the capacitor has a low durability. In addition, since the negative electrode of the capacitor contains lithium, the positive electrode (polarized electrode) of the capacitor already has a voltage of about 3 V even in uncharged condition. As described in Examples of the Japanese Patent Application Laid-open (KOKAI) No. 8-107048(1996), in the case where the capacitor is charged up to 4.3 V, the change in potential from that in the uncharged state is about 1.3 V. Therefore, when such an element is used as a capacitor, the energy density thereof is lower than that of ordinary capacitors.
For these reasons, it has been demanded to provide a high energy density electric double-layer capacitor not only having a positive electrode charging potential lower than the decomposition voltage of an electrolyte used, but also exhibiting a large potential difference between charging and discharging, e.g., not less than 3 V.
As a result of the present inventors' earnest studies for solving the above-mentioned problems, it has been found that by optionally controlling a rest potential of electrodes by a method which is essentially different from conventionally proposed methods such as a method of preventing the decomposition of electrolyte or a method of reducing the amount of impurities contained in electrodes, it can become possible to apply to a capacitor a wide range of voltage which nearly reaches the decomposition voltage of electrolyte due to oxidation or reduction thereof, thereby enabling an energy density of the capacitor to be increased notwithstanding the capacitance thereof remains uncharged. The present invention has been attained on the basis of this finding.